Indirect-recognizing antitrapping systems for adjusting devices in motor vehicles are known from DE 30 43 118 A1, EP 0 359 853 B1, DE 43 15 637 A1 and DE 197 11 979 A1, for example. These systems are called indirect-recognizing because an object being trapped is not recognized directly at the location at which it happens (such as when using an electric terminal board laid along the closing contour), but rather as a result of the reaction of this occurrence via the adjusting system's power chain on the drive, which is operatively connected to a sensor. In some types of embodiment, the motor's drive shaft has an annular magnet mounted on it which generates signals in an associated Hall sensor which are in turn assessed in an electronic control apparatus. Should an instance of trapping arise then it can be expected that the adjusting movement of the adjustable part (e.g. a window pane) will be characteristically slowed down, which also slows down the drive via the mechanical coupling. This extends the time periods between two successive (Hall) signals. If a stipulated limit value is exceeded, the drive is stopped by the control device and if appropriate the direction of rotation is reversed in order to resolve the trapped condition again.
Such antitrapping or injury prevention systems are in use for adjustment kinematics for window lifters, in which the gear ratio between the drive and the adjustable part (window pane) remains constant. One example of this is single-strand or double-strand cord window lifters with an essentially linear track, as disclosed by way of example in DE 198 02 478 A1 and WO 98/50658. In principle, any alteration in the dynamics of the adjusting process or with respect to the forces occurring which is recognized by the electronic injury prevention system needs to be associated with an external action in this case, and therefore—when prescribable limit values are exceeded—an instance of trapping must be inferred.